Elucidation of advanced function by combined high-resolution neutron and X-ray analysis

Tamada, Taro; Kinoshita, Takayoshi*; Tada, Toshiji*; Kuroki, Ryota

To help resolve long-standing questions regarding the catalytic activity of the serine proteases the structure of porcine pancreatic elastase has been analyzed by high-resolution neutron (1.65 resolution) and X-ray (0.94 resolution) crystallography. In order to mimic the tetrahedral transition intermediate, a peptidic inhibitor was used. The neutron and X-ray data show that the hydrogen bond between His57 and Asp102 is not consistent with a low-barrier hydrogen which is predicted to have the hydrogen midway between the donor and acceptor atom. The neutron analysis also shows that the oxygen of the oxopropyl group of the inhibitor is present as an oxygen anion rather than a hydroxyl group, supporting the role of the "oxyanion hole" in stabilizing the tetrahedral intermediate in catalysis.